The efficiency of semiconductor light-emitting diodes (LEDs) has continuously improved since the first fabrication of an infrared device in 1960. The key metric has been the external quantum efficiency (EQE), which describes the number of photons externally radiated per electron injected into the device. The external quantum efficiency is the product of the injection efficiency, internal quantum efficiency, and extraction efficiency.
The injection efficiency of an LED describes the fraction of electrons that are injected into the active region relative to the number of electrons flowing into the active region. The internal quantum efficiency describes the ratio of photons generated in the active region by radiative transitions relative to the sum all radiative and non-radiative processes, and can be improved by increasing the rate of radiative transitions or by decreasing the rate of non-radiative transitions, e.g., by introducing additional radiative states or states that couple to radiative states.
The extraction efficiency of an LED describes the fraction of photons that escape the device relative to the number of photons generated in the active region. When the electron-hole pairs generated in the active region of an LED decay, they can either decay radiatively by emitting a photon and thus producing light, or can decay non-radiatively as dissipated energy in the crystal lattice, for example by producing heat. In a simple planar LED, the light is emitted in an “escape cone” whose properties are defined by the LED's angle of total internal reflection at the semiconductor/atmosphere interface. The remainder of the light is trapped within the LED structure by total internal reflection and can only escape at the ends of the semiconductor slab making up the LED or through some imperfection in the device surface.
A major vein of LED research is directed to enhancing the extraction efficiency of light from the semiconductor into the surrounding atmosphere by modifying the optical structure in or around the semiconductor host.
Some researchers have employed a microcavity in the LED to guide the preferential propagation of generated photons into the light extraction cone of the LED. See M. Mastro, J. Caldwell, R. Holm, R. Henry, and C. Eddy Jr., “Design of Gallium Nitride Resonant Cavity Light-Emitting Diodes on Si Substrates,” Adv. Mater. 2008, 20, 115-118.
Others have used a photonic crystal deposited on an LED to diffract and redirect the light out of the semiconductor and into the atmosphere. See M. Mastro, C. Kim, M. Kim, J. Caldwell, R. Holm, I. Vurgaftman, J. Kim, C. R. Eddy, Jr., and J. Meyer, “Zinc Sulphide Overlayer Two-Dimensional Photonic Crystal for Enhanced Extraction of Light from a Micro Cavity Light-Emitting Diode” Japanese Journal of Applied Physics, Vol. 47, No. 10, 2008, pp. 7827-7830.
Still others have attempted to improve the extraction of light from an LED by roughening a surface of the semiconductor structure, thus scattering the emitted photons and enhancing the light extraction efficiency. See B. Kim, M. Mastro, H. Jung, H. Kim, S. Kim, R. Holm, J. Hite, C. Eddy Jr., J. Bang, and J. Kim, “Inductively coupled plasma etching of nano-patterned sapphire for flip-chip GaN light emitting diode applications” Thin Solid Films 516 (2008) 7744-7747; and B. J. Kim, H. Jung, J. Shin, M. Mastro, C. Eddy Jr., J. Hite, S. Kim, J. Bang, and J. Kim, “Enhancement of light extraction efficiency of ultraviolet light emitting diodes by patterning of SiO2 nanosphere arrays, Thin Solid Films 517 (2009) 2742-2744.
Another approach to improving light extraction from an LED has been to invert the structure of the LED itself. In a traditional GaN-based LED structure, the final grown layer is a p-type GaN contact layer to avoid magnesium memory effects and doping-generated growth defects. Use of low-resistance Ohmic contacts to the p-type region requires a high level of magnesium doping in the p-type contact layer. To minimize the growth of and contacting to the p+ GaN contact layer, an inverted LED structure has been suggested by various commercial entities. See, e.g., U.S. Pat. No. 7,170,097 to Edmond et al., entitled “Inverted Light Emitting Diode on Conductive Substrate”; and T. Takeuchi, G. Hasnain, S. Corzine, M. Heuschen, R. Schneider, Jr., C. Kocot, M. Blomqvist, Y. Chang, D. Lefforge, M. Krames, L. Cook, and S. Stockman, “GaN-Based Light Emitting Diodes with Tunnel Junctions,” Jpn. J. Appl. Phys. Vol. 40 (2001) pp. L 861-L 863.
Operation of such an inverted LED device is similar to operation of a standard LED in that electron and holes from n- and p-type regions, respectively, are injected into a quantum well active region. However, in most cases an inverted LED structure has both Ohmic contacts made to n-type material. This creates an npn semiconductor that would prohibit current flow in either the forward or reverse direction for non-degenerate doping levels. Such an inverted LED, biased by two n-type contacts, will often include a (degenerate) n++/p++ tunnel junction that under local reverse bias allows electrons to quantum-mechanically tunnel through a thin depletion region into the p-type layer adjacent to the active region. At this point, the operation is similar to a traditional LED where holes are injected into the active region from the adjacent p-type layer and electrons are injected from the adjacent n-type layer into the active region, where the electrons and holes may recombine radiatively.
Still another approach has found that introduction of a plasmonic metal in the near vicinity (tens of nanometers) of the semiconductor active region can improve the emission of light from an LED.
A surface plasmon (SP) from such plasmonic metals at a perfectly flat metal/semiconductor interface is in the form of a non-propagating evanescent wave. If the SP is not coupled to the excitons (i.e., the electron-hole pairs) formed in the active quantum well (QW) region, any emitted photon can be non-radiatively absorbed into the metal surface and dissipated as heat. See C. Langhammer, Z. Yuan, I. Zorić, and B. Kasemo, “Plasmonic Properties of Supported Pt and Pd Nanostructures,” Nano Letters 2006 Vol. 6, No. 4 833-838. However, when the excitons within the active QW region and the SPs within the plasmonic metal are coupled, the energy in the exciton transitions into the surface plasmon state in the metal or at the metal/dielectric (which is usually a semiconductor) interface. The surface plasmon will oscillate or propagate for a short time but eventually either will dissipate as heat via a phonon or will scatter as a photon in the air and be emitted from the device. Thus, by coupling the SPs and the excitons, an alternative decay route for the excitons is created, which improves the probability that the exciton will decay into an SP and scatter, thus produce light. K. W. Liu, Y. D. Tang, C. X. Cong, T. C. Sum, A. C. H. Huan, Z. X. Shen, Li Wang, F. Y. Jiang, X. W. Sun and H. D. Sun, Appl. Phys. Lett., 94, 151102 (2009). This coupling occurs when the energy of the electron-hole pair has sufficient overlap in energy with the energy of the surface plasmon (SP) resonance condition.
The QW must be in close proximity to the plasmonic metal in order for the QW excitons to be located within the SP fringing field so that they may be coupled into the SP state. Okamoto et al. calculated the fringing field depth as 47 and 77 nm for silver and aluminum, respectively. See K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A. Scherer, “Surface-plasmon-enhanced light emitters based on InGaN quantum wells,” Nature Materials, Vol. 3, No. 9, pp. 601-605 (2004).
Many approaches have been taken to improve the SP-exciton coupling and enhance the emission of scattered light. For example, Okamoto et al. found large enhancements in the emission from InGaN quantum well (QW) devices when a thin film of platinum or silver was deposited 10 nm above the QW, relying on imperfections and roughness in the metal coating to couple the surface plasmons into scattered light. See Okamoto, supra.
Another approach involves structuring the plasmonic metal as nanoparticles. Such structuring allows for tuning of the SP resonance condition where the SP density of states (DOS) is extremely high, to closely match the peak emission of the active layer of the semiconductor emitter. In addition, the periodicity of the metal nanoparticles allows a more controlled scattering of the SPs into radiative photons. See A. Neogi and H. Morkoç, “Resonant surface plasmon-induced modification of photoluminescence from GaN/AlN quantum dots,” Nanotechnology 15 (2004) 1252-1255, who deposited arrays of silver nanoparticles onto a structure with GaN quantum dots located in close proximity to the surface.
However, as noted above, in order to achieve coupling of the exciton and the SP, the plasmonic metal must be situated very close, usually within 30 nm, to the active QW of the LED device. This necessity presents a subtle dilemma to construction of a functional plasmonic enhanced (In,Al)GaN LED. In a traditional GaN-based LED structure, use of low-resistance Ohmic contacts to the p-type region is based on magnesium doping in the p-type contact layer. However, -magnesium is a deep acceptor and cannot produce a high level of activated holes in (In,Al)GaN. The low hole concentration in the traditional p-type GaN top contact layer requires a thickness of at least 200 nm to achieve proper current spreading. In addition, a thin, i.e., less than 80 nm, p-type (In,Al)GaN top contact layer would exhibit band bending from the surface states that would hamper the injection of holes into the active region.
The magnesium dopant also has a high activation energy, which limits the density of active acceptors to approximately 1018cm−3 at room temperature. For a simple pn junction, this corresponds to a p side depletion width of 32.2 nm and an n side (with 5×1018cm−3 active donors) depletion width of 6.5 nm. Once depletion from the surface is included, the necessary theoretical thickness of the p-type region is already beyond the SP fringing field depth and thus coupling will not occur. In practice, the high level of defects also results in low carrier mobility and thus high resistivity of the p-GaN, which necessitates a thick (100 to 250 nm) current-spreading p-type layer in commercial LEDs, which only exacerbates this issue.
Kwon et al. attempted to avoid this issue by interrupting the growth, placing the silver nanoparticles internal to the semiconductor, and then continuing the growth; specifically, the plasmonic metal layer was situated in the n-GaN within a few nanometers of the multi-QW. See M. Kwon, J. Kim, B. Kim, I. Park, C. Cho, C. Byeon, and S. Park, “Surface-Plasmon-Enhanced Light-Emitting Diodes” Adv. Mater. 2008, 20, 1253-1257. Although Kwon et al. maintained that the semiconductor crystal was not perturbed by the discontinuous silver interlayer, any foreign particle would very likely disturb the crystallographic stacking during epitaxy, and SPs of the silver interlayer that do scatter are emitted as photons within the semiconductor slab and so will suffer from extraction issues (e.g., total internal reflection) that are problematic for all LEDs.
Thus, it would be advantageous to place the plasmonic metal exterior to the semiconductor slab making up the LED so that the SP-exciton energy couples from the active region to the metal, thus avoiding the necessity to exit via the semiconductor/air light cone.